Liquid center thread wound golf ball

Abstract: Disclosed is a thread wound golf ball comprising a liquid center comprising a rubber bag containing a liquid, a rubber thread layer provided on the outside of the liquid center and a cover for covering the rubber thread layer, wherein the moment of inertia of the golf ball measured at about 23°C is in the range of from 75 to 80 g.cm² and the rate of increase of the moment of inertia measured at about -30°C to that measured at about 23°C is within about 2%.

Description: The present invention relates to a thread wound golf ball having a liquid center which is composed of a rubber bag containing a liquid. Such a ball is known from GB-A-483977. In general, professional golfers or advanced golfers like thread wound golf balls because of their high impact resilience, good shot feel and excellent spin performances. However, it has hitherto been considered that thread wound golf balls are not suitable for amateur golfers who require a long flight distance because their flight distance is inferior in comparison with two-piece golf balls. In respect of thread wound golf balls, since the amount of back spin applied to the golf ball when hit is large, a lifting power is liable to act on the golf ball. In the process immediately after launching at which the velocity of the golf ball is high, flight power and lift power of the golf ball is strong and a golf ball which is launched at a low launching angle is blown up. The velocity of this golf ball is gradually reduced due to air pressure or resistance during flight. At this stage, the spinning ratio is reduced and the lifting force of the golf ball is reduced. Accordingly, the trajectory of the golf ball begins to drop rapidly as the lifting force and projectile force decrease. That is, the landing angle thereof is large and the rolling distance thereof is small. This is the cause of low flight distance in comparison with two-piece golf balls. In the ideal golf ball, the launch angle is high, the initial velocity is large, the amount of spin is small and a suitable lifting force acts on the golf ball. Therefore, the golf ball reaches the peak point gently without blowing up, and then begins to drop. It is preferred that the amount of spin is large (i.e. the lifting force is large) and the golf ball is not easily dropped at this time. Heretofore, the weight of golf balls has been concentrated in the center part thereof to reduce the moment of inertia such that spin is easily applied tothe golf ball. The diameter of the center of the golf ball is merely defined as 25 to 29 mm empirically, as described in "Rubber Industry Handbook". The moment of inertia is small and spin is easily applied to the golf ball, and there is a limit in flight distance. Therefore, the thread wound golf ball which can accomplish a long flight distance has hitherto been required. In this case, it is considered that the initial velocity of the golf ball can be increased by employing rubber thread having high impact resilience. However, the initial velocity of the golf ball is defined by rules of United States Golf Association, and there is a limit in initial velocity. Further, as described in Japanese Laid-Open Patent Publication 59-129072, there is suggested a trial of increasing flight distance by decreasing the specific gravity of the center and, in contrast, increasing the specific gravity of the cover to increase the moment of inertia. However, due to the excessive amount of fillers which are formulated in the cover material, stiffness of the cover itself becomes high, which results in hard shot feel, and this is not preferred. Although the moment of inertia is increased,it is limited. Further, this technique applies to solid center balls. In order to reduce the amount of spin appropriately, a liquid center is employed. The liquid center has hitherto been employed for balata cover golf balls to which spin is easily applied. The liquid center itself exhibits no impact resilience but it follows deformation of the golf ball on hitting. The deformation sendsforcesto the rubber thread around the center, thereby affording restoring forces to the rubber thread having high impact resilience. In Japanese Laid-Open Patent Publication No. 60-92782, it is described that it is effective to decrease the spin ratio of the golf ball by increasing the diameter from the conventional diameter (25.4 to 26.99 mm) to 28.58 mm. Since the amount of spin of golf balls with balata covers is large, there have been trials to increase the flight distance thereof controlling the amount of spin. However, this technique merely discloses a change of the diameter of the center within a narrow range such as 25 to 29 mm. As to the diameter of the center, in Japanese Laid-Open Patent Publication No. 54-4626, it is described that the desirable diameter of the center is 25 to 29 mm. In Japanese Laid-Open Patent Publication Nos. 59-129072 and 48-4025, it is described that the desirable diameter of the center is 30 to 38 mm and 23 to 39 mm, respectively. In "Rubber Industry Handbook" issued on 1973, page 864, column of golf ball, it is described that the outer diameter of the liquid center is 25 to 29 mm. In Japanese Laid-Open Patent Publication No. 61-187875, there is suggested a golf ball comprising a liquid center having a specific gravity of 0.7 to 1.3 and a center cover having a specific gravity larger than that of the liquid center, the diameter of the liquid center being 25 to 34 mm. In this golf ball, the center cover has a large specific gravity in order to increase the moment of inertia. On the other hand, in Japanese Laid-Open Patent Publication No. 2-255162, it is suggested that the specific gravity of the center cover is decreased and, as a result, the golf ball becomes soft because the amount of the filler of the center cover is decreased, thereby affording large deformation of the center and reducing the amount of back spin when the ball is hit in order to increase the flight distance. However, the flight distance was not greatly increased in any golf ball. The main object of the present invention is to provide a rubber thread wound golf ball having improved characteristics. This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description. The present invention provides a rubber thread wound golf ball comprising a liquid center composed of a rubber bag containing a liquid, a rubber thread layer provided on the outside of the liquid center and a cover for covering the rubber thread layer, wherein the moment of inertia of the golf ball measured at about 23°C is in the range of from 75 to 80 g.cm 2 and the rate of increase of the moment of inertia measured at about -30°C to that measured at about 23°C is within 2%. That is, the present invention provides a golf ball with a liquid center which contributes to deformation of the ball as liquid upon hitting the ball, and when the ball rotates by backspin after launching, contributes to the moment of inertia as a part of the golf ball, like a solid center. The golf ball thus has large moment of inertia and relatively little spin. That is, the golf ball flies without blowing up during a period from the launching point to the peak point because of its structure and material and spin is not easily applied thereto. During the period from the peak point to the landing point, damping of spin is small because the moment of inertia is large. Therefore, the lifting force is comparatively large and the golf ball does not easily drop , thereby increasing the flight distance. That is, the golf ball of the present invention is considered to be greatly improved and approaching an approximately ideal golf ball. When the liquid center is compared with the solid center, it is preferable to consider how much they contribute to the moment of inertia. That is, with respect to solid centers, no large change arises at a temperature from about -30°C to about room temperature. However, the liquid center freezes at a temperature lower than the freezing point because of its water content. That is, it can take both solid and liquid forms depending on the temperature to which it is exposed. When the content of the liquid center is a perfect liquid, it does not greatly contribute to the moment of inertia (a value of the moment of inertia is small) in case of spinning of ball, It does contribute to the moment of inertia (a value of the moment of inertia is large) at the frozen state. That is, the fact that the liquid state and frozen state are greatly distinct in moment of inertia means that the liquid center at the liquid state does not contribute to the moment of inertia under the condition of the temperature used for playing golf. It is necessary for the golf ball that the moment of inertia is large within a temperature range for playing golf without increasing the specific gravity of the cover of the golf ball (i.e. without deteriorating shot feel ), as a matter of course. For this purpose, the center, in which the weight is concentrated, plays a significant role. In general, an air layer is partially present in the rubber thread layer since the rubber thread is not tightly wound in comparison with a solid golf ball (e.g. a one-piece golf ball as a mass of rubber or a two-piece golf ball prepared by coating the mass with the cover). Therefore, the specific gravity of this part of the ball is small in comparison with the solid golf ball. There is a limit to the increase in specific gravity of the cover because the shot feel deteriorates. Therefore, it is necessary to make up the weight at the center part. In order to improve flight performances substantially, it is necessary to use a liquid center on which spin is not easily applied instead of a solid center on which spin is easily applied. Furthermore, it is important that the moment of inertia at room temperature is large such that damping of spin becomes small. In a golf ball which satisfies the above conditions, moment of inertia measured at about 23°C is 75 to 80 g·cm 2 , and a rate of increase of a moment of inertia measured at about -30°C, at which the liquid center is frozen, to that measured at about 23°C is within about 2%. The moment of inertia can be measured by a normal measuring instrument. In the present invention, it is measured by a moment of inertia measuring instrument, model number 005-002, commercially available from Inertia Dynamics Inc. When the golf ball does not satisfy the above moment of inertia, the excellent and improved advantage of the present invention is not obtained. As the center solution comprising the liquid center of such a golf ball, for example, those comprising 100 parts by weight of water, in the range of from 5 to 20 parts by weight of a freezing-point depressant (e.g. glycerin, ethylene glycol, etc.), in the range of from 50 to 100 parts by weight of a filler (e.g. barium sulphate, etc.) and in the range of from 10 to 30 parts by weight of a viscosity modifier (e.g. clay, etc.) are preferred. As a matter of course, any center solution may be used if the resulting golf ball satisfies the above performances, but the center solution mainly composed of water is substantially preferred. As the rubber bag containing the center solution of the liquid center, there can be used those which have hitherto been used for liquid centers of thread wound golf balls. For example, it can be formed by formulating in the range of from 1 to 10 parts by weight of zinc oxide and in the range of from 10 to 50 parts by weight of a filler (e.g. calcium carbonate, barium sulphate, etc.) in 100 parts by weight of natural rubber. Pouring of the center solution into the rubber bag can be conducted by methods which are hitherto known. For example, there can be used a method of penetrating a needle in a rubber bag, a method comprising freezing a center solution in advance and then covering the frozen center solution with a rubber, a method of bonding two semi-spherical shell rubbers in a center solution, etc. By winding a rubber thread for golf balls on the liquid center thus obtained, a thread wound center composed of a center and a rubber thread layer thereon is formed. The rubber thread is normally composed of a rubber component of natural rubber and synthetic isoprene rubber (mixing ratio is in the range of from 75:25 to 25:75). As a matter of course, any rubber thread which has been used for the rubber thread for known golf balls may be used. The rubber thread thus obtained is covered with a cover mainly composed of an ionomer resin or balata (transpolyisoprene), and dimples are provided thereon to obtain a golf ball. As described above, according to the present invention, there is provided a golf ball with a liquid center having large moment of inertia and little spin, which contributes to deformation of ball as liquid when the ball is hit, and when the ball rotates by backspin after launching, contributes itself to the moment of inertia as a part of the golf ball, like a solid center. That is, the golf ball flies without blowing up during a period from the launching point to the peak point because of its structure and material wherein spin is not easily applied. During a period from the peak point to the landing point, damping of spin is little because the moment of inertia is large. Therefore, the lifting force is comparatively large and the golf ball does not easily drop , thereby increasing the flight distance. That is, the golf ball of the present invention is considered to be greatly improved and approaching an approximately ideal golf ball. EXAMPLES The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Examples 1 to 2 and Comparative Examples 1 to 2 After a liquid center shown in Table 1 was formed, the liquid center was coated with a center bag, followed by winding a normal rubber thread to form a rubber thread center. Then, the rubber thread center was coated with a cover resin shown in Table 1 to form a rubber thread wound golf ball. Example No. Comparative Example No. 1 2 1 2 Center Center solution Water 100 100 100 100 Glycerin 13 13 17 5 Barium sulfate 80 85 155 - Clay 17 13 22 - Center bag NR (natural rubber) 100 100 100 100 Zinc white 5 5 5 75 Calcium carbonate 25 25 25 165 Sulfur 2 2 2 1 Vulcanization accelerator 3 3 3 2 Cover resin Transpolyisoprene 100 100 100 100 NR (natural rubber) 10 10 10 10 Zinc white/titanium oxide 5/15 5/15 5/15 5/15 Sulfur/vulcanization accelerator 1.5/1.5 1.5/1.5 1.5/1.5 1.5/1.5 Characteristics (e.g. specific gravity, and thickness, diameter and weight of the center cover) of the liquid centers of the resulting golf balls, the diameter of the thread wound cores of the thread wound centers, characteristics (e.g. weight, hardness, moment of inertia at 23°C, moment of inertia at -30°C and rate of increase in moment of inertia) of the golf balls and the number of dimples are shown in Table 2. Further, characteristics (e.g. ball initial velocity, spin, carry and total) of the golf balls were measured by subjecting the resulting golf ball to a hitting test at a head speed of 45 m/second. The results are also shown in Table 2. Example No. Comparative Example No. 1 2 1 2 Characteristics of center Specific gravity of center solution 1.50 1.56 1.78 1.0 Thickness of center cover (mm) 1.7 1.6 1.7 2.5 Specific gravity of center cover 1.1 1.1 1.1 2.1 Diameter (mm) 30.1 29.5 26.4 28.8 Weight (g) 18.5 18.0 14.4 17.6 Core Diameter of thread wound core (inch) 1.605 1.605 1.595 1.605 Ball Weight 45.3 45.3 45.4 45.3 Hardness Reasonable Reasonable Reasonable Reasonable Moment of inertia at 23°C 76.9 76.7 74.5 74.7 Moment of inertia at -30°C 77.5 77.2 75.1 79.0 Rate of increase in moment of inertia 0.8 0.7 0.7 5.7 The number of dimple 400 432 360 392 Measurement values at head speed of 45 m/second Ball initial velocity (m/s) 63.4 63.5 63.5 63.5 Spin (r.p.m) 3110 3105 3375 3360 Carry (yard) 232.5 231.9 227.3 228.7 Total (yard) 249.3 248.8 242.9 243.7 Regarding Comparative Example 1, a conventional liquid center is used and the diameter of the center is comparatively small and, further, the moment of inertia is small and the amount of spin is large. Therefore, the flight distance is not good in comparison with Examples 1 and 2. Regarding Comparative Example 2, the liquid center containing a center solution having a specific gravity of 1.0 mainly composed of water is used and the diameter of the center is comparatively large and, further, the center cover has a comparatively large specific gravity so that the moment of inertia became large, but is not equal to the moment of inertia of Examples 1 and 2. Besides, there is a great difference in the moment of inertia between the frozen state and the state at room temperature. That is, the golf ball of Comparative Example 2 has a large structural factor for increasing the moment of inertia. However, it is found that the center solution of this liquid center does not greatly contribute to the moment of inertia at the liquid state. Therefore, it is necessary to think out some device to obtain the moment of inertia which is the same as that of the center solution at the solid state (frozen state), even at room temperature. In comparison with these Comparative Examples 1 and 2, Examples 1 and 2 plays a role in increasing the moment of inertia in view of structure and material of center solution and, therefore, spin is not easily applied to the golf ball and the flight performances are excellent. That is, it is necessary that the specific gravity of the center solution is 1.3 to 1.6, the thickness of the center cover is 1.5 to 2.0 mm, the specific gravity of the center cover is 0.95 to 1.2 and the weight of the center is 17 to 20 g, preferably 18 to 19 g. Further, it is preferred that the diameter of the center is not 25 to 29 mm (conventional value) but not less than 29 mm (preferably 29.5 to 32 mm). The moment of inertia is not less than 75 g·cm 2 , preferably not less than 76 g·cm 2 . The above conditions make flight performances of the golf ball good. In the golf ball of the present invention, the center solution is composed of 100 parts by weight of water, 5 to 20 parts by weight of glycerin or ethylene glycol, 50 to 100 parts by weight of barium sulphate and 10 to 30 parts by weight of clay. The center bag is composed of 100 parts by weight of natural rubber, 1 to 10 parts by weight of zinc oxide and 10 to 50 parts by weight of a filler (e.g. calcium carbonate, barium sulphate etc.). The rubber thread is composed of natural rubber/isoprene rubber (75:25 to 25:75). The cover resin is composed of 100 parts by weight of transpolyisoprene resin, 3 to 20 parts by weight of natural rubber, 0 to 20 parts by weight of high-styrene resin, 2 to 15 parts by weight of zinc oxide and 5 to 25 parts by weight of titanium oxide.

Claims: A thread wound golf ball comprising a liquid center comprising a rubber bag containing a liquid, a rubber thread layer provided on the outside of the liquid center and a cover for covering the rubber thread layer, characterised in that the moment of inertia of the golf ball measured at about 23°C is in the range of from 75 to 80 g.cm 2 and the rate of increase of the moment of inertia measured at about -30°C to that measured at about 23°C is within about 2%. A thread wound golf ball as claimed in claim 1, wherein the weight of the liquid center is in the range of from 17 to 20 g. A thread wound golf ball as claimed in claim 1, or claim 2 wherein the specific gravity of the center liquid of the liquid center is in the range of from 1.3 to 1.6. A thread wound golf ball as claimed in any one of claims 1 to 3, wherein the rubber bag has an average thickness in the range of from 1.5 to 2.0 mm and a specific gravity in the range of from 0.95 to 1.2. A thread wound golf ball as claimed in any one of claims 1 to 4, wherein the center liquid comprises 100 parts by weight of water, 5 to 20 parts by weight of glycerin or ethylene glycol, 50 to 100 parts by weight of barium sulphate and 10 to 30 parts by weight of clay. A thread wound golf ball as claimed in any one of claims 1 to 5, wherein the rubber bag is prepared from a rubber composition comprising 100 parts by weight of natural rubber, 1 to 10 parts by weight of zinc oxide and 10 to 50 parts by weight of a filler. A thread wound golf ball as claimed in any one of claims 1 to 6, wherein the rubber thread is prepared from a rubber composition comprising natural rubber and isoprene rubber in an amount ratio in the range of from 75:25 to 25:75 of natural rubber/isoprene rubber. A thread wound golf ball as claimed in any one of claims 1 to 7, wherein the cover is composed of 100 parts by weight of transpolyisoprene resin, 3 to 20 parts by weight of natural rubber, 0 to 20 parts by weight of high styrene resin, 2 to 15 parts by weight of zinc oxide and 5 to 25 parts by weight of titanium oxide.

Inventors: EBISUNO TADAHIRO, MORIYAMA KEIJI, SUGIMOTO KAZUSHIGE, YAMADA MIKIO, EBISUNO, TADAHIRO, MORIYAMA, KEIJI, SUGIMOTO, KAZUSHIGE, YAMADA, MIKIO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37